The invention relates generally to the molding of moldable substances and, more particularly, to the molding of mixtures which contain a fibrous material and a binder for the latter.
In the manufacture of chip boards, which contain a heat-setting binder, it is known to additionally heat the "fleece", that is, the mixture of fibrous material and binder, either immediately prior to or during the hot-molding operation by means of a high-frequency electrical field. This heating, which is in addition to the conventional contact heating, is intended to accelerate the condensation process. For the purpose of the additional heating, it is possible, for example, to employ the sheets upon which the mixture is distributed and upon which it is inserted into the molding press as electrodes. In this manner, the mixture may be subjected to the influence of a high-frequency field until it has set. Generally, heating is also carried out by conventional contact heating simultaneously. One disadvantage of this method resides in the fact that it is extremely complicated since the sheets upon which the mixture is distributed must be insulated from the metallic parts of the molding press. In addition, it is difficult to achieve satisfactory insulation and the procedures for overcoming the insulation difficulties make this method very expensive. Moreover, by proceeding in the manner described above, the mixture is subjected to a high-frequency field only during the hot-molding operation and this method thus accelerates only the hot-molding operation. Finally, the only advantage achieved by this method, which is an acceleration of the hot-molding operation, in no manner compensates for its high cost.
It has also been proposed to place the fleece on a sheet, for example, and then, outside of the mold and before inserting the fleece therein, to subject the latter to the influence of a high-frequency field in order to preheat it to such an extent that the condensation process is initiated. The preheated fleece is then placed in the mold and immediately compressed, with the final setting of the mixture being effected by contact heating. This method is intended to better utilize the mold by permitting the preheating for the next batch of the mixture to be molded to be carried out while setting of the mixture already in the mold occurs. In this manner, the time for which the mold is occupied by any given batch of the mixture is reduced. However, great insulation difficulties arise with this method also and the necessity for obtaining satisfactory insulation, which latter may be achieved only with great difficulty, make the apparatus for carrying out this method extraordinarily complicated.
In general, the known methods and apparatus described above, have only two purposes. On the one hand, they are intended to accelerate setting of the binder by additionally heating the mixture of fibrous material and binder either during or immediately prior to the hot-molding operation. On the other hand, they are intended to effect better utilization of the molds or molding tools. However, these methods and apparatus are neither intended to nor are capable of improving the quality of the final product. Moreover, these methods and apparatus are not intended for utilization in the production of preformed articles by cold-molding procedures.